1. Field of the Invention
This invention relates in general to series resonating circuits and more particularly to an antenna arrangement formed by a ferrite loop antenna in series with a single loop antenna.
2. Background Discussion
In portable radio and especially in paging equipment and related applications, size is an extremely important factor. In the past, many portable paging devices employed an antenna for receiving purposes which tended to significantly increase the overall dimensions of the device of which it is a part which may be a prohibiting factor especially when compactness is a primary consideration. At the same time, no degradation in performance can be tolerated to any large extent. This is particularly so with respect to antenna apparatus. High gain response is most desirable and, indeed, critical for the full range capabilities of the paging receivers to be realized. However, because of size limitations, the associated antenna arrangement cannot take the usual form of high gain antenna configurations conventionally encountered in paging devices.
In the past, a typical paging receiver has been normally worn on the side of the body when in use, usually attached to the belt or in a pocket. However, there has been greater importance placed on decreasing the size of the receiver in an aesthetic pleasing package. Unlike past paging receivers, a wrist carried paging receiver is strapped around the wrist of the paging user, similar to a watch. Thus, a wrist carried paging receiver is used in many different body positions. It may be held in front of the face, on the side of the body, outstretched from the human body, or placed on a conducting or non-conducting surface. Thus, for a wrist carried paging receiver, the overall sensitivity must be designed to maximize reception in all possible body positions.
Furthermore, a paging receiver designed to be carried by the wrist, is even more sensitive to size and antenna configuration. Wrist carried radio sets in the past as shown in U.S. Pat. No. 3,032,651 to Gisiger-Stahi et al, have incorporated supertime loop antennas or simple loop type antennas which are typically responsive to the magnetic field component of the transmitted RF signal. This type of antenna system provided marginal performance but was desirable because the loop antenna could be concealed within the wrist band housing and responded well when the receiver was in close proximity to a human body. However, the overall sensitivity of the antenna receiver combination was somewhat degraded when the receiver was not in close proximity to a human body.
Another way to achieve an effective compromise between gain and size factor was to employ a ferrite rod antenna for a paging receiver This is a magnetic antenna structure as contrasted to the usual electric antenna structure. As is known, magnetic antenna devices of this sort are usually in the form of a rod or cylinder, a ferrite material on which a spiral conductor is wound. The spiral conductor forms a completely closed loop, and the ferrite core serves to concentrate the magnetic lines of flux and thereby induce an appropriate voltage signal in the enclosed loop configuration.
For a wrist carried paging receiver, the conventional ferrite antenna structure nevertheless has a number of disadvantages, particularly when operating at relatively high frequencies is required, say in the UHF frequency range, problems in effectively resonating the antenna apparatus are experienced. Too many turns of conductor on the ferrite core results in excessive inductance which in turn makes the necessary capacitance for resonance impractical. That is, the value of the capacitance becomes inordinately small. Reducing the number of turns to obtain a more practical value for the resonating capacitor, however, reduces the level of induced voltage and thus is counterproductive with respect to gain.
Moreover, at the higher frequencies, hand capacity, while a relatively low value, nevertheless presents an alternate or parallel path for sufficiently low impedance to significantly increase the loss factor with respect to the antenna device as a whole. The term "hand capacity" in this regard generally denotes one capacitative effect produced by the close proximity of the antenna device to any part of the human body. Furthermore, the ferrite loop antenna produces a highly directional polarization field response which exhibits poor sensitivity when the loop axis is quadrature to the direction of the transmitting signal.
Thus, it would be highly desirable to provide an antenna system which satisfies the aesthetic size problems associated with a wrist carried paging receiver and avoids the undesirable effect of changing sensitivity when the orientation of the wrist carried paging receiver is changed.